The southern High Plains of Colorado, an area of about 2,800 square miles in the southeastern part of the state, is underlain by the Ogallala Formation of late Tertiary age, The southern High Plains of Colorado extend from the Colorado State line on the east and the ans south to the edge of the Ogallala Formation on the north and west. The Ogallala Formation is an unconsolidated or partly consolidated seposit of sand, gravel, clay silt, and caliche.